SU836538A1 - Hot-wire gauge - Google Patents

Description

(54) HEAT MANOMETER

.,. ,,. one ,. :. : The invention relates to the field of measurement technology, in particular to thermal vacuum gauges. Known thermal gauges consist of a heater in the form of a tape and a temperature-sensitive element, such as a thermocouple, the junction of which is welded to the current-heated tape .l. The disadvantage of such gauges is low sensitivity, significant measurement error, difficulty in implementing a remote measurement system due to the low level of the output signal of the thermocouple carrying: useful information. The closest in technical essence to the present invention is a thermal pressure gauge containing pressure sensors with heating and temperature-sensitive elements, connected to a measuring circuit. A quartz element jp is used as a thermosensitive element. As a heater, using a metal track applied along the periphery of a quartz element. A quartz element with a heating element is structurally made in the form of a sensor connected to a measuring circuit, including the circuit for the task of heating element current, an amplifier with positive feedback, and a measuring device. The quartz element is connected to the amplifier with a positive feedback and is used as the frequency-generating element of the oscillator. When the measured pressure changes, the temperature of the heater and the quartz element changes, which leads to a change in the frequency of the oscillator. This change in the frequency of the output signal is functionally related to the measured pressure 2. The drawback of this type of heat gauge is that the measurement accuracy is low because the temperature sensitive quartz jr-element responds equally both to the change in the heater temperature and to the change in ambient temperature as well as a large thermal inertia, since the mass of the Kjartsev element is much larger than the mass of the heating element and to achieve thermal equilibrium of the newly established The values of the measured pressure trebu- a long time. Such a pressure gauge cannot be used to measure relatively rapidly changing pressures.

The aim of the invention is to improve the accuracy of pressure measurement and reduce the thermal inertia of the pressure gauge;

This goal is achieved by the fact that in a thermal manometer containing an pressure sensor with a sensing element placed in a housing and a measuring circuit including a heating current circuit with series-connected stabilized constant voltage source, an adjusting resistance and an ammeter, an amplifier with positive feedback and output meter, a bracket mounted in the housing is inserted into the pressure sensor, and the sensing element is made in the form of a metal string placed in the air A magnetic excitation system of its oscillations, with the ends of the strings attached to the supports of the bracket and connected to the amplifier input, while the heating current circuit is parallel to the amplifier input and contains a series choke connected in series with a constant voltage source.

In order to reduce the temperature error in a manometer, the pressure sensor can be equipped with an additional string, identical to the first one, the ends of which are fixed on additional stands, the string is placed in the air gap of the additional magnetic excitation system of its oscillations. In this case, the strings are located symmetrically with respect to the axis of symmetry of the sensor body, and a mixer and a second amplifier with positive feedback, to the input of which an additional string is connected, are inserted into the measuring circuit, and the outputs of the amplifiers are connected through the mixer to the output measuring device.

FIG. 1 shows the sensor of the thermal pressure gauge; a section, in FIG. 2, 3, and 4 — sections A – A, B – B, B – B; in fig. 5 - measuring circuit thermal gauge.

The thermal gauge sensor consists of body 1 (Fig. 1), inside which is mounted a bracket 3 on a threaded rod 2. Racks 4 and 5 are rigidly fixed to bracket 3. A bar b is fixed to the plane of rack 5. The ends of the rack 4 and slat b lie in the same plane. The ends of the string 7, having. the initial elastic deformation of tension, fixed on racks 4 and 5, bracket 3 by means of linings 8 and 9 i String 7 is placed in the air gap of the magnetic oscillation excitation system 10 (Fig. 2) rigidly mounted on bracket 3. End

the strings 7, mounted on a stand 4, are electrically connected to the sensor body 1. In order to implement a string auto-oscillator with a magneto-electrically driven self-oscillation method, the second end of the string 7 is isolated from the sensor body 1 by means of a mica pad 11 (Fig. 3) and mica washers 12 and 13, underneath the fastening screws 14, 15. Metal washers 16 and 17 serve to more evenly distribute the forces acting on mica washers 12 and 13 when tightening the screws 14 and 15. There is an air gap between the body of the screws 14 and 15 and the holes in the bar B. The screws 18 serve to press the pads 8 and 9 while fixing the ends of the string 7. The end of the string 7, isolated from the housing 1, is connected to the electrical pressure lead 19 (Fig. 1). The electrical output of the second end of the string 7 is removed from the sensor body. The housing 1 is closed by a cover 20.

The ends of the string 7 are connected to the input terminals a and b of the electronic amplifier 21 with a positive feedback connection (Fig. 5). The bridge circuit, in one of the arms of which string 7 is included, is part of the input stage of the electronic amplifier 21. Parallel to the input AB of the amplifier 21, a current problem is connected that provides heating of the string 7. This circuit consists of consecutive thrusts Dp, a stabilized source constant voltage U,. variable resistor Rp, measuring resistance Ra, and ammeter mA. Choke OP is necessary in order to avoid the effect of string 7 shunting over alternating current (pumping current) by the task circuit of the heating current. The variable resistor ftp provides adjustment of the heating current of the string 7. The heating current can be controlled either by measuring the voltage drop across the measuring resistance R with a compensator, either a digital voltmeter, or a WA ammeter.

After balancing the bridge by changing the value of the balancing resistance Rg and turning on the supply voltage of the amplifier 21, the string 7 together with the amplifier forms a string auto-oscillator whose frequency is determined by the temperature of the string 7. The oscillation amplitude of the string 7 is adjusted by adjusting the positive feedback current by changing resistor resistance. wasps - the output of the amplifier 21 electric frequency-modulated signal can be applied to a digital measuring device 22 or an analog measuring device 23. As a digital device 22 any standard, electronic counter frequency meter can be used. Heat gauge works as follows. In the absence of a heating current, the frequency of the string oscillator is determined by the initial tension of the string 7. After the heating current is applied from a stabilized constant voltage source, the string 7 is heated and its longitudinal tension decreases, which leads to a decrease in the frequency of the oscillator. As the pressure of the gas return medium decreases, the heat removal from the string 7 into the environment due to the heat conductivity decreases and the temperature of the string 7 increases. With increasing temperature, the force of the longitudinal tension of the string 7 and the frequency of the string oscillator decrease. With the constant voltage of the source of direct current heating the frequency of the stringer autogen generator will be uniquely associated with the functional dependence measured by pressure. The frequency of the string autogenerator at normal atmospheric pressure mainly depends on the tension of the string 7 and, to a first approximation, can be determined by the expression - (1) where fjj is the initial frequency of oscillation of the string autogenerator at normal atmospheric pressure (9.81-10 Pa) ; Fg is the force of the longitudinal tension of the string 7; io - tensile stress in a string 7 8p - string length 7; S is the cross-sectional area of the string 7 ;. J is the density of the string material. In accordance with Hooke's law, the value of the initial elastic g is the shape of the string 7,, le,, (2) where dHpu is the initial elastic deformation of the string 7; E is the elastic modulus of the second kind of material of the string 7. From where: (3) expression (1) will take into account 2Eo P In After the constant TQK I is omitted from string 7, string 7 will warm up and its temperature will exceed the ambient temperature by the amount, i L. o ° co-oii, the initial elastic deformation of the string 7 dECh; decreases by, the value of, for tcroL l to u dL is the temperature coefficient of linear expansion of the material of the string 7. The oscillation frequency of the string oscillator after turning on the heating current, taking into account equations (4) and (6) - J Е (de -) / 74 Y - Modulus change elasticity when the temperature of the string 7 varies with an accuracy quite acceptable for engineering calculations is neglected due to smallness. The equation of heat balance of the string 7 heated by current, taking into account the fact that most of the thermal energy is dissipated due to the thermal conductivity of the gas, and the thermal conductivity through the converter elements, as well as by convection and radiation, is negligible, looks like (Vt, o) | (ie-too), 3c where 3 is a warm-up current flowing along string 7; R - active resistance: strings 7; X - coefficient of thermal conductivity of gas, the pressure of which. changes; a is a coefficient depending on the geometric dimensions of the string 7 and the sensor body 1; t is the current temperature value of string 1; djj - internal diameter of the sensor housing 1) dj. - the diameter of the string 7. At a gas density corresponding to the atmospheric pressure region, the thermal conductivity of gas% is almost independent of its density, t. does not depend on pressure change. As the pressure in the internal cavity of the sensor decreases, the mean free path of the gas molecules becomes in the same order as the distance between the string 7 and the walls of the sensor housing 1. In this case, the thermal conductivity of a gas is determined by the number of remaining molecules and depends mainly on the pressure of the gas (its density), and not on its temperature. Based on the above, it can be assumed that in the region of relatively low pressures, the thermal conductivity coefficient is functionally associated with the pressure pressure (P). where P is the gas pressure in the sensor housing 1. From expression (8), taking into account (9), we will have At tc- t Ф (Pla Pola in expression (10) all the values are constant and denoted by black K, we get D VTFT. Reduction of the elastic deformation of the string 7, due to the decrease in gas pressure Lr l o-dL & t. EoflL cpf, (12) Frequency of a string auto-oscillator, taking into account an increase in the temperature of string 7 while decreasing the measured pressure il - fr- (d & x), (1з Because of the selected LYu modes - cfKo const, then -. with -epV. PG RoJi ad erg H sLVO H (P) 0-1 where K--2. - With a consistently selected constructive sensor parameters, the value ... L. lies, in the range e 0.1-0.3. Representing the irrational term of expression (14) in the form of a row, we get g / - g Ka 1 V /, - l i oi (-24 (P) e- WTJ Restricting by the second member depending on the measured pressure, the difference between the initial value of the frequency fg and the current value of f D -f fai-faib With an exactly known dependency functional and known geometric parameters of the transducer, the function of converting such thermal manum ra can be determined by calculation. However, the functional dependence for various gases is determined with usually experimentally with limited accuracy. In this connection, calibration characteristics are used in practice for working with heat gauges. In the case under consideration, this will be an experimentally determined dependence of the frequency of the string oscillator on the value of the measured pressure. FIG. b shows a sensor of a thermal manometer with two strings, a slit / in FIG. 7 - measuring circuit. The thermal gauge sensor contains an additional string 24 placed in the air gap of an additional magnetic excitation system 25 of its oscillations, the strings 7 and 24 are located symmetrically with respect to the axis of symmetry of the sensor housing 1. Additional string 24 (thermo-compensation) is connected to the input a b of the electronic amplifier 26 with a positive feedback connection and, together with it, forms the second self-oscillator (Fig. 7). Unlike the first oscillator (string 7 and amplifier 21), string 24 does not allow heating current to flow and the frequency of the oscillator is determined only by the ambient temperature and temperature coefficients of linear expansion of the materials of string 24 and bracket 3. The outputs of amplifiers 21 and 26 are connected to inputs a frequency mixer 27. The output of mixer 27 is connected to the inputs of analog 28 and digital 29 measuring instruments, measuring the difference frequency allocated by the mixer .27. When operating in the frequency ratio measurement mode, signals from points c and d of the measuring circuit (Fig. 7) can be fed to the inputs A and B of a standard digital electronic counting frequency meter of any type. Heat gauge works as follows. With a pressure of 9.81 "Yu Pa (760 mmHg) and a heating current flowing through one of the strings, the frequency of the oscillators ff. and f (; 2 are equal, i.e. fQ3 f and the output of the mixer 27 is the difference frequency ff 01 is zero. When the pressure in the gas in the converter housing 1 decreases, the string 7 through which the heating current is passed increases its temperature as the thermal conductivity of the rarefied gas decreases An increase in the temperature of the string 7 leads to a decrease in the magnitude of its elastic deformation and to a reduction in the frequency of the string oscillator to f. The difference in frequencies at the mixer output, {foa - fj f 0 - ft, will be functionally associated with the measured pressure. different The temperature coefficients of the linear expansion of the materials of the bracket 3 and strings 7 and 24 change the frequencies of the string oscillators as the ambient temperature changes. But since these changes will be identical, then the output of the mixer 27 will have a temperature error of zero heat gauge that will be compensated for reduces the total value of the temperature error by approximately one order of magnitude. By analogy with the above, the temperature error is compensated also when the measuring circuit operates in the mode of measuring the ratio of the frequencies of string autogenerators. String 7, which plays the role of a heater of a rygo and thermosensitive element in the absence of a heating current, has such an initial elastic deformation of the Zkeni so that the frequency of its own transverse oscillations at normal atmospheric pressure (9.81–10 Pa) fj would be higher than the frequency of its own transverse oscillations thermal compensation string 24 fo2, i.e. When the rated heating current flows, the frequency of natural transverse oscillations of the first string 7 decreases to f due to the appearance of temperature deforming, reducing the value of the initial elastic deformation. In this case, it is possible to ensure the equality of the oscillation frequencies of both strings (17). The frequency of natural transverse oscillations of the first string 7 can be determined in accordance with the well-known string equation, due to the heating current flowing to it. Accordingly, the frequency of the natural transverse oscillations of the second thermal sensation string 24 Е Д Е (П U f01 is the initial elastic deformation of the string 24 .. To satisfy equality (17), it is necessary that t (d Е Еоав this construction, the geometrical dimensions of both strings 7 and 24 are materials, ., from which they are made, are the same. Strictly speaking, the value of the modulus of elasticity in expression (18) will be slightly less than in expression (19), due to the fact that the temperature of the first string 7 is higher. However, due to the smallness of this difference, the elastic modulus When the temperature of the first string 7 is changed, they will be neglected. Such an assumption for engineering calculations in the case under consideration is quite acceptable. With a precisely known functional dependence and known geometric parameters of the sensor, the conversion function of such a heat gauge can be determined by calculation. Since the dependences of R (P) are significantly different from experimental, then in practice, with a view to improving the accuracy of pressure measurements, calibration characteristics are used. In the case under consideration, this will be an experiment.) 1 the dependence of the variability (ratio) of the frequencies of two string autogenerators, obtained at the mixer output, on the measured pressure. D as a digital measuring instrument in the proposed thermal pressure gauge can be used a standard electron-counting frequency meter of any type, and as an analog measuring instrument any capacitor frequency meter with a pointer indicating instrument. The principle of temperature compensation is as follows. When the ambient temperature changes, the temperature of the sensor housing 1 changes. After some time, the temperature of the bracket 3 and strings 7 and 24 will also change. With the temperature coefficients of linear expansion of the material of bracket 3 and strings 7 and 24 changing perfectly perfectly, the frequency of string oscillators will not change and the pressure measurement error due to changes in the temperature of case 1 virtually absent. However, in practice, there will always be some difference in the temperature coefficients of linear expansions of the materials of strings 7 and 24 and of bracket 3. In this case, a change in the temperature of body 1, bracket 3 and strings 7 and 24, due to a change in ambient temperature, will change string frequency oscillators. Suppose that when the temperature changes, the frequency of the first stringed auto generator also changed and in accordance with (15) its value can be written as

, R (i i)

pj ().

(20) If the value of the measured pressure was estimated by the frequency of the first string 7, then as can be seen from (14), the measurement error would be the sum of the error zero ± d f and the sensitivity sensitivity 1 jfb) 2, the rune exceeds the knowledge .0.05-0.15, then the maximum s-sensitivity error of sensitivity is 7–20 times less than the error of zero. Therefore, if there was a possibility to compensate for the temperature error, zero, then the total measurement error due to a change in the ambient temperature would be reduced by a factor of 7–20. For this purpose, a thermal gauge string 24 was introduced into the design of the heat gauge. Since it is made of the same material, the string and the first (heated current) string changes as the ambient temperature changes and, as a result, the temperature of the body 1 of the sensor, bracket 3 and the string 24 itself, the frequency of the second oscillator is changed by the same value u.fj. and will be - equal to 02Г 01 i ft. Then, in accordance with expression (16), the frequency difference between the strings 7 and 24 at the output of the mixer 27 is determined by subtracting expression (14) from (15), i.e. (five) . d .- (± d) .Ч (Р) р. ) From the last two expressions, it follows that the introduction of the temperature-compensating string 24 makes it possible to completely eliminate the temperature error zero Af. This reduces the total value of the temperature error of the string heat gauge, depending on the selected structural parameters of the sensor, by 7–20 times.

If the measuring circuit operates in the ratio measurement mode; the frequencies of two string oscillators corresponding to the choice of the initial frequencies of the strings f and f can be obtained with the analog effect of reducing the temperature error. . 1. Thermal gauge comprising a pressure sensor with a sensing element placed in the housing and a measuring circuit including a heating current circuit connected in series with a stabilized constant voltage source, a control resistance and an ammeter, an amplifier with positive feedback and an OUTPUT measuring device characterized in that, s. In order to improve the accuracy of pressure measurement and reduce thermal inertia, a bracket fixed in the housing is inserted into the pressure sensor, and the sensing element is designed as a metal string placed into the air gap of the magnetic excitation system of its oscillations, with the ends of the string fixed on the bracket stands and connected to the input an amplifier, wherein the current heating circuit is connected in parallel to the input of the amplifier and contains a choke connected in series with a source of direct voltage. 2.Thermal iviaHOMeTp according to claim 1, different from the fact that, in order to reduce temperature error, the pressure sensor is equipped with an additional magnetic excitation system and an additional string, identical to the first, the ends of which are fixed on additional racks, the string is placed in the air gap of the additional magnetic excitation system of oscillations, while the strings are located symmetrically relative to the axis. symmetry of the sensor body, and a mixer and a second amplifier with a positive reverse a bond, which is connected to the input of the additional string, wherein the amplifier outputs are connected through a mixer to the output measuring device. Sources of information taken into account in the examination 1.Turitsin A.M. et al. Electrical measurements of non-electrical quantities ... L., Energie, 1975, p. 360. 2. The author's certificate of the USSR. № 513283, cl. G 01 L 21/10, 1974 (prototype).

FIG. 2

FIG.

FIG. five

..

YY KX, 4 Y Y / 1 I

/ y / .., L

. FIG. 7 1 L7 1 // I u- / f / I

Claims (2)

The claims (21) ^f 02i ~ ^f 0i - ^Δ / Then, in accordance with expression (16), the frequency difference of the strings 7 and 24 at the output of the mixer 27 is determined by subtracting expression (14) from (15), i.e. ^X ^ 0Г ^Л ^) ⁼ 2.ч> (Р) ^£ 01 * 2ЧХР) ' ^Δί 17 (22) or _Δ £ χ - ^ - (£ ο-ι ^{± Δ} ^) · ^ ^ίΡ) Γ ¹ (23) I From the last two expressions it blows that the introduction of the temperature compensation string 24 allows us to completely eliminate the temperature error of the zero Afi · This reduces the total value of the temperature error of the string thermal pressure gauge, depending on the selected design parameters of the sensor, by 7–20 times. next. <q 1. A thermal manometer containing a pressure sensor with a sensing element in the housing and a measuring circuit that includes a heating current circuit with a stabilized constant voltage source connected in series, an adjustable resistance and an ammeter, a positive feedback amplifier and an output measuring device, It is due to the fact that, in order to increase the accuracy of pressure measurement and reduce thermal inertia, an arm mounted in the housing is introduced into the pressure sensor, and the sensitive element is made in the form of a metal string placed in the air gap of the magnetic system for exciting its vibrations, the ends of the strings being fixed on the bracket posts and connected to the amplifier input, while the current heating circuit is connected in parallel with the amplifier’s input and contains the source 3 constant voltage choke. 2. The thermal pressure gauge according to claim 1, distinguished by the fact that, in order to reduce the temperature error, the pressure sensor is equipped with an additional magnetic system 40 the excitation oscillations and extra string identical to the first ends of which are secured to additional racks, the string is placed in the air gap of the magnetic system further oscillation excitation, the string raspolozhony symmetrically with respect osi.simmetrii The ratio _Y of the sensor housing, and the measuring circuit incorporated mixer and the second amplifier with positive feedback, to the input of which an additional string is connected, and the outputs of the amplifiers are connected through the mixer to the output measuring device.